By way of example, an hydraulic circuit of the type with which the present invention is concerned is used in the agricultural planter disclosed in my co-pending application entitled "Agricultural Planter", U.S. Ser. No. 779,164. The planter disclosed in that application is a large agricultural planter having a lift frame which may be elevated relative to a carrier frame to two different positions of elevation. In the first or lower elevated position, the lift frame raises the individual planter units out of the planting position so that the planter may turn at the end of a field. This first height for the lift frame not only removes the individual planter row units from engagement with the ground, but it disengages the drive system for the seed meters. Otherwise, the meters would continue to meter out the seed, fertilizer and insecticide.
The hydraulic circuit for raising the lift frame to this first position includes a master cylinder cooperating with at least one slave cylinder. Actually, there are two such hydraulic circuits in the planter, one on the right side and one on the left side, but they are similar in structure and they function together.
When the hydraulic cylinders elevate the lift frame to the end turn position, the slave cylinders have reached full extension, but the master cylinder has not. The master cylinder may be further extended to elevate the entire lift frame (in which case, the wings of the lift frame are locked to the center section, and the master cylinder is further extended) to a second, higher position. At the second limit position of the master cylinder, the entire lift frame may be rotated 90.degree. about a vertical axis so that it extends parallel to the direction of travel of the tractor for road transport.
It will thus be apparent that the farmer raises the planter to the first limit position of the master cylinder (i.e., for an end turn) many more times than he raises the master cylinder to its second limit position for road transport.
A conventional type of hydraulic control valve in the tractor (sometimes called the "main valve" or "selective control valve") used by the farmer for these control functions has a mechanical detent for maintaining the operating lever in the "up" position, until a sharp increase in pressure, as occurs when the slave cylinders reach the limits of their extension, is sensed by the control valve to kick the lever out of the detent position and lock up the hydraulic system at that time. However, for a number of reasons, and for certain types of hydraulic systems, depending upon the manufacturer of the tractor, for example, operation of the hydraulic system has not reliably disengaged the operating lever from the detent at the first limit position for end turns. If disengagement of the control valve at the first limit position does not occur reliably, the operator must continually direct his attention to the planter to ensure that it does not elevate to the road transport position. Thus, it is important to actuate the selective control valve and release it from detent in a reliable manner when the master cylinder has reached its first limit position; and the present invention has, as a principal objective, that purpose.
The present invention provides a separate auxiliary discharge conduit with a solenoid-actuated control valve having a normally closed position. The auxiliary discharge conduit is located in the hydraulic circuit so as to permit the master cylinder to extend from its first limit position to its second limit position only when the solenoid valve is opened. Thus, when the main control valve is placed in detent the first time, the master and slave cylinders extend until the slave cylinders are fully extended. Conventional rephasing ports at the rod end of the slave cylinders are eliminated, so that when the slave cylinders are fully extended, and the auxiliary discharge conduit is closed, pressure increases rapidly and the resulting pressure "spike" releases the main control valve from its detent position. This, of course, disconnects the hydraulic pump from the cylinders and locks the hydraulic cylinders in place.
If the operator wants the master cylinder to extend from its first limit position to a second limit position, he opens the solenoid-actuated valve and again operates the main control valve to extend the master cylinder. During this operation, the slave cylinders remain fully extended, and hydraulic fluid is discharged from the rod end of the master cylinder through the auxiliary discharge conduit because the solenoid-actuated valve is open; and the hydraulic fluid returns to the hydraulic reservoir or the return port of the pump.
Thus, the system insures that the selective control valve on the tractor will be released from detent when the slave cylinders are fully extended at the first limit position of the master cylinder so that the operator need not be concerned when he raises the planter for an end turn. Yet the master cylinder can be extended even further to a second limit position when desired to elevate the lift frame to the transport position.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein like reference numerals will refer to similar elements in the various views.